Human O.sup.6 -guanine alkyltransferase ("GATase") is a suicide enzyme that irreversibly transfers an alkyl adduct from the O.sup.6 -position of guanine onto a cysteine residue in its active site. O.sup.6 -alkylguanine-DNA alkyltransferase is responsible for the repair of mutagenic lesions induced by simple methylating agents, as well as for the repair of precursors of cytotoxic interstrand cross-links which are induced by chloroethylating anticancer drugs. The enzyme is of growing interest in cancer chemotherapy research because of its ability to suppress formation in vitro of chlorethylnitrosourea (CENU)-induced DNA interstrand cross-links, which are considered to be the cytotoxic lesions generated by these agents. Consistent with these observations, the susceptibility of human cancer cell lines and human tumor xenografts to CENU therapy correlates with their levels of GATase activity.
About 20% of cultured human tumor cell lines, termed Mer.sup.31, appear deficient in GATase activity and are hypersensitive to the lethal effects of CENUs and agents that induce O.sup.6 -methylguanine in DNA. By contrast, cells derived from normal tissues do not display such hypersensitivity, suggesting the opportunity for selective therapy for patients with the Mer.sup.- (methyl repair minus) subset of tumors.
The presence of Mer.sup.- tumors in patients remains to be established. Wiestler et al. (Carcinogenesis 15:121-124(1984)) found in a series of 23 human tumor biopsy specimens, that all contained measurable O.sup.6 -guanine alkyltransferase activity. This failure to identify Mer.sup.- tumors could, however, merely reflect contamination of the specimens by normal host stroma cells, or it may indicate that the Mer.sup.- phenotype is not a property of human tumors in situ, but arises during adaptation to tissue culture or the establishment of xenografts.
Resolution of these questions requires both sequence and structural analysis of the O.sup.6 -guanine alkyltransferase, studies which have been greatly impaired, if not entirely prohibited, by the inability to purify the GATase protein to homogeniety. Additionally, conventional methods have, to date, proved unsuccessful in raising monoclonal antibodies ("MAbs") to the subject protein, thus further restricting the use of and research relating to the human GATase enzyme.